This invention relates to rail fastening devices for railway systems, and more particularly it relates to a protective device for electrically insulating a rail fastener from its supporting structure.
Tracks or rails for modern rapid transit rail systems are generally fastened to a supporting structure by means of a plurality of spaced apart rail fasteners. Examples of such rail fasteners that have been used are described in U.S. Pat. Nos. 3,784,097 to Landis and 3,858,804 to Hixson. Both of these assemblies comprise a metal top plate which is fastened to the rail and a base plate separated from the top plate by a layer of elastomeric, a material to provide vibration isolation and a degree of electrical insulation, with the entire assembly being fastened to supporting structure such as a concrete bed.
Because rapid transit rails are used as electrical conductors for traction power current as well as for train speed command signals, it is necessary to provide and maintain electrical insulation between the rails and the rail support structure. The aforesaid rail fasteners presently in use do provide some electrical insulation between the rails and the rail support structure. However, heretofore, the surface creepage paths provided by the insulating elements of existing apparatus were found to be relatively short and easily contaminated with dirt and rail wear products. When these contaminated surfaces then became wet by fog, rain, or ground water, electrically conductive paths were formed over which electrical leakage currents flowed. Such leakage currents caused corrosion of the rail as well as of the metal parts of the rail fasteners and supports, resulting in further contamination of the surface creepage paths. This additional contamination of the creepage paths resulted in further reduction of the electrical resistance of the creepage paths and thence in larger magnitudes of leakage currents.
Such excessive leakage currents from train operation over poorly insulated rails caused destructive corrosion of rail, rail fasteners, rail support structures, metal tunnel liners, and other metallic structures. An excessively low rail-to-rail support structure resistance caused by such corrosion also tended to short out train speed command signals between the rails. In such a situation, the shorted section of track then appeared to the train speed command system as though it were occupied by a train, and train operation was disrupted.
To prevent loss of electrical train speed command signals and leakage of electrical traction currents over creepage paths, rail circuit insulation integrity must be maintained, and the rail fastener or support insulation must provide electrical insulation even when wet and contaminated with electrolyte. Therefore, the rail must be insulated from the rail fastener or support with electrical insulation means which provides relatively long surface creepage paths. Such long creepage paths will maximize the electrical leakage path resistance between rail and rail support even when the rail and the rail fastener or support apparatus are wet and contaminated with electrolyte.
In addition, the rail support insulating device must not interfere with the rail fastener's ability to securely fasten the rail relative to the support structure and to limit relative movement of the rail to within acceptable tolerances in the vertical, lateral, and longitudinal directions.
Accordingly, a general object of the present invention is to provide a rail insulating device that solves the aforesaid problems by electrically insulating rapid transit rails from their support structures.
Another object of the invention is to provide a device that reduces the flow of leakage current on a rail fastener for a rapid transit rail system, thereby greatly reducing corrosion and deterioration of the fastener and the rail.
Another object of the present invention is to provide a rail insulating device for electrically insulating rapid transit rails from their support structures that provides long surface electrical creepage paths for leakage current so that the device will maintain its electrical insulating properties when wet and contaminated with electrolyte.
Still another object of the present invention is to provide a rail insulating device that can be applied to any type of rail fastening method that clamps the rail in such a way that neither the rail hold-down assembly nor the rail lateral restraint device will make point contact against or penetrate the device.
Yet another object of the present invention is to provide a device for reducing the leakage current and thus the corrosion rate on a rail fastener which is relatively easy and economical to install on an existing rail fastener without requiring its modification or removal.